Self-Sensitized Cobalt Molecular Photocatalysts: The Effects of Alkali Cations on Tuning the Activity of H ₂ Production

This study presents a series of cobalt-alizarin complexes, L6Co2Li3(TBA)3 (LCoLi), L6Co2Na3(TBA)3 (LCoNa), and L3CoK(TBA)2 (LCoK), where L is an alizarin dianion, and TBA is a tetrabutyl ammonium cation, as highly active molecular photocatalysts for H2 production under visible light. The precious-metal-free complexes feature anthraquinone-based dyes as light-absorbing ligands, redox-active cobalt centers for proton reduction, and redox-inactive alkali cations in proximity for tuning the catalytic activity. Photocatalytic H2 evolution is significantly enhanced when employing different alkali metal centers in the complex in the order of LCoLi > LCoNa > LCoK. Under optimal conditions, LCoLi achieves a turnover number (TON) of 5680, a turnover frequency (TOF) of 59.7 h–1, and a quantum yield (QY) of 0.35% at 450 nm, representing state-of-the-art photocatalysts in molecular systems. Mechanistic studies reveal that tuning the alkali Lewis-acidic metal cations in the vicinity of the alizarin and Co centers facilitates electron transfer from the electron donor to the photocatalysts, thus realizing efficient H2 evolution. This report provides an effective strategy for the synthesis of highly efficient self-sensitized photocatalysts.